Fe-rich layered oxide cathode for sodium-ion batteries enabled by synergistic modulation of ion transport and structural stability

Abstract

The sustainability and availability of raw materials are of critical importance for sodium-ion batteries (SIBs) to have competitiveness. Iron (Fe) as an inexpensive and electrochemically active element in SIBs layered cathodes offers unique advantages. Nonetheless, Fe-rich materials typically perform poor and most reports focus on materials with Fe content around 1/3, as higher Fe content leads to Jahn-Teller distortion, irreversible structure damage, transition metal (TM) migration, and poor air stability. Herein, for the first time we report an Fe-rich material (Fe = 0.5) that has high energy density (143.28 mA h g⁻¹ in 2–4 V) and shows comparable cyclability with typical low-Fe materials through the synergistic modulation of ion transport and structural stability. The pillar effect of Ca in the Na layer limits the gliding of the TMO₂ slab and the migration of TM ions, while the addition of Al enhances the TM(3deg*)-O(2p) hybridization, reduces the lattice distortion, and suppresses the undesired phase transition. In a sodium-ion full cell system, an excellent cyclability of 82 % capacity retention after 150 cycles can be achieved, while the unmodified Fe-rich cathode only shows a capacity retention of 38 %. This work firstly demonstrates the feasibility of using Fe-rich materials as cathode materials for SIBs.